People think I'm a very calm, collected person when, In reality, I've failed one of my classes, I haven't done my taxes yet, I'm cheating on my boyfriend, and I regularly fantasize about hurting people, physically and emotionally, just because they irritate me to no end. I'm a failure but they don't believe me when I tell them.

.... failing one of your classes is not the end of the world. So is not doing your taxes.... Cheating on your boyfriend, however, is kind of a big thing. Fantasizing about hurting people is not that big of a deal, as long as you don't actually hurt anyone. Don't judge yourself for your impulses or feelings. But judge yourself over your actions: cheating on someone is not okay. And talk to someone who believes you. Either find better friends, or talk to a professional.